ISOFLURANE - Detailed Pharmacology Notes

1. CHEMISTRY AND PHYSICAL PROPERTIES

• Isoflurane is a halogenated methyl ethyl ether - a structural isomer of enflurane
• Chemical formula: CHF₂-O-CHClCF₃ (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether)
• It is the most potent of the volatile anesthetics currently in clinical use

Chemical structures of inhaled anesthetic agents. Note isoflurane is a halogenated methyl ethyl ether with a chlorine atom on the alpha-ethyl component. (Goodman & Gilman)

Blood:gas solubility intermediate between halothane (2.5) and sevoflurane (0.65). This means induction and recovery are faster than halothane but slower than sevoflurane or desflurane. - Barash Clinical Anesthesia 9e

2. PHARMACOKINETICS

A. Uptake (FA/FI Ratio)

Rise of alveolar (FA) toward inspired (FI) concentration. Isoflurane (orange) rises more slowly than nitrous oxide, desflurane, and sevoflurane, but faster than halothane. (Goodman & Gilman / Barash)

• Inspired concentration - higher concentration speeds FA/FI rise (concentration effect)
• Alveolar ventilation - higher minute ventilation speeds delivery; important for isoflurane
• Blood:gas partition coefficient - isoflurane (1.46) has intermediate uptake into blood
• Cardiac output - high CO increases blood uptake, slowing FA rise
• Alveolar-venous partial pressure difference - greatest at induction

• Inhalational induction: 1.5-3% in O₂ for <10 minutes
• Maintenance: 1-2% (approximately 1-2 MAC)
• Adjuncts (opioids, N₂O) reduce the required isoflurane concentration

B. Distribution

• At equilibrium: P(CNS) = P(blood) = P(alveolar)
• Brain:blood coefficient = 2.2-2.6 → at equilibrium, brain contains ~2.2x the quantity of isoflurane as the same volume of blood
• High fat solubility (fat:blood = 45) leads to significant accumulation in adipose tissue over long procedures

C. Metabolism and Elimination

• >99% eliminated unchanged via the lungs - primary route of clearance
• Only 0.17% is recovered as metabolites (among the lowest of all potent volatile anesthetics)
• Minor hepatic metabolism via CYP2E1 produces trifluoroacetic acid (TFA)
• Small amounts of inorganic fluoride are generated, but serum fluoride levels are clinically insignificant
• No nephrotoxicity - unlike methoxyflurane or enflurane
• No hepatotoxicity - TFA production is minimal; immune-mediated hepatitis is extremely rare
• Not a mutagen, teratogen, or carcinogen

Because isoflurane undergoes little metabolism, it is considered nontoxic to the liver and kidney. - Lippincott Pharmacology

3. MECHANISM OF ACTION

A. CNS Depression Mechanisms

• Isoflurane potentiates GABA-A receptor (ligand-gated Cl⁻ channel) function
• Binds to hydrophobic pockets within the receptor's transmembrane domains
• Increases frequency and duration of Cl⁻ channel opening → hyperpolarization → neuronal inhibition
• This is the major mechanism of unconsciousness and sedation

• Isoflurane inhibits NMDA (N-methyl-D-aspartate) glutamate receptors
• Reduces excitatory glutamatergic neurotransmission
• Contributes to immobility, analgesia, and amnesia

• Activates TREK-1 and TASK background K⁺ channels
• Increased K⁺ conductance hyperpolarizes neurons
• Contributes to CNS depression

• Inhibits HCN1 channels (the molecular basis of Ih current in neurons)
• Reduces the pacemaker current that normally keeps neurons in a state of readiness

• Reduces Na⁺ channel activity, decreasing action potential generation and propagation
• Inhibits voltage-gated Ca²⁺ channels in nerve terminals, reducing neurotransmitter release

• Immobility (MAC) is mediated predominantly at the spinal cord level (suppression of reflex arcs)
• Unconsciousness and amnesia are mediated at cortical and subcortical brain levels
• Different molecular targets may dominate at each anatomic site

B. The Lipid Solubility Correlation

4. MINIMUM ALVEOLAR CONCENTRATION (MAC)

• Increasing age (MAC declines by ~6%/decade after 40 years)
• Hypothermia
• Hypotension, anemia, hypoxia
• Other CNS depressants (opioids, benzodiazepines, propofol, N₂O)
• Pregnancy
• Hypothyroidism, metabolic acidosis

• Hyperthermia
• Chronic alcohol use, CNS stimulant use
• Hyperthyroidism
• Children (MAC peaks in infancy, then declines)

• MAC is lower in neonates, peaks in infancy, then declines with age
• More potent than sevoflurane but pungent odor makes inhalational induction unacceptable for children
• Miller's Anesthesia 10e

5. EFFECTS ON ORGAN SYSTEMS

A. Cardiovascular System

• Hypotension: Concentration-dependent decrease in arterial blood pressure - the primary dose-limiting effect
• Mechanism of hypotension: Predominantly decreased systemic vascular resistance (vasodilation in skin and muscle); cardiac output is relatively well maintained
• Heart rate: Mildly elevated heart rate as baroreceptor-mediated compensation for hypotension; rapid increases in concentration cause transient tachycardia and hypertension due to sympathetic stimulation
• Cardiac output: Maintained (unlike halothane which directly depresses the myocardium); only minimal left ventricular depression in vivo
• Baroreceptor function: Significantly attenuated but partial preservation of carotid baroreflexes (better than halothane)
• Coronary vasodilation: Isoflurane is a potent coronary vasodilator - simultaneously increases coronary blood flow AND decreases myocardial O₂ consumption (favorable balance)
• Coronary steal controversy: Dilation of normal coronary arteries could theoretically divert blood away from territories supplied by stenotic vessels (coronary steal). In clinical practice, this has proven to be a rare occurrence - Barash 9e, Goodman & Gilman
• Hypotension can be treated with a direct-acting vasoconstrictor such as phenylephrine

B. Respiratory System

• Dose-dependent respiratory depression - decreases tidal volume; tachypnea is less pronounced compared to other volatile agents, so net minute ventilation falls more
• Hypercapnia develops at concentrations >1 MAC (elevated PaCO₂)
• Hypoxic and hypercapnic drive blunted - even at 0.1 MAC, isoflurane blunts the normal ventilatory response to hypoxia and hypercapnia
• Airway irritation: Pungent odor stimulates upper airway reflexes → breath-holding, coughing, laryngospasm, salivation (prevents use for mask induction)
• Bronchodilation: Isoflurane is a bronchodilator (clinically useful in asthmatic patients), though perhaps not as potent as halothane

C. Central Nervous System (CNS)

• Cerebral metabolic rate (CMRO₂): Reduced - isoflurane decreases cerebral oxygen requirements
• EEG: Progressive slowing; at 2 MAC, produces an electrically silent EEG (burst suppression), reducing CMRO₂ by ~50%
• Cerebral blood flow (CBF): Dilates cerebral vasculature → increases CBF at >1 MAC
• Intracranial pressure (ICP): May increase in patients with poor intracranial compliance; these effects are less pronounced than halothane
• Hyperventilation: The vasoconstricting response to hypocapnia is preserved during isoflurane anesthesia; ICP increases can be prevented by hyperventilation. Unlike halothane, hyperventilation does NOT need to be instituted before isoflurane is started - Morgan & Mikhail 7e
• Neuroprotection: Some evidence of anesthetic preconditioning; the EEG silence at 2 MAC is used therapeutically in status epilepticus

D. Neuromuscular System

• Skeletal muscle relaxation - direct effect
• Potentiates both non-depolarizing and depolarizing neuromuscular blocking agents
• Allows dose reduction of neuromuscular blockers during general anesthesia

E. Renal System

• Decreases renal blood flow, glomerular filtration rate (GFR), and urinary output
• However, no clinically significant nephrotoxicity - minimal metabolism means minimal fluoride generation

F. Hepatic System

• Reduces total hepatic blood flow (hepatic arterial + portal venous)
• However, hepatic oxygen supply is better maintained than with halothane, because hepatic arterial perfusion is preserved
• Liver function tests usually unaffected - Morgan & Mikhail 7e
• Immune-mediated hepatitis (analogous to halothane hepatitis) is extremely rare due to minimal TFA production

G. Uterus

• Produces dose-dependent uterine relaxation (tocolysis)
• At ≥1 MAC may impair uterine contraction and increase blood loss during obstetric procedures

6. CLINICAL USE

Indications

• Maintenance of anesthesia (primary role) - after induction with IV agents (propofol, thiopentone)
• Preferred for longer surgical procedures due to low cost (more economical than sevoflurane or desflurane)
• Status epilepticus refractory to other treatments (produces EEG burst suppression at 2 MAC)
• Useful in asthmatic patients (bronchodilator property)

Why NOT used for Inhalational Induction

• Pungent, ethereal odor causes breath-holding, coughing, laryngospasm, and excessive secretions
• Sevoflurane (sweet-smelling, non-pungent) is preferred for mask induction, especially in children

Advantages over Halothane

• Greater cardiovascular stability (maintains cardiac output via SVR reduction vs. direct myocardial depression with halothane)
• Much less hepatotoxic (0.17% vs ~20% metabolism; no reliable immune hepatitis)
• Better maintenance of hepatic arterial perfusion
• "Gold standard" volatile anesthetic since introduced in the 1970s - Barash Clinical Anesthesia 9e

Position Among Volatile Agents (Comparison Summary)

7. BIOTRANSFORMATION AND TOXICITY

• Hepatic CYP2E1 oxidation → Trifluoroacetic acid (TFA) + small quantities of inorganic fluoride
• Only 0.17% of absorbed isoflurane undergoes hepatic biotransformation

• 99% exhaled unchanged

• TFA production is minimal compared to halothane; immune-mediated hepatic necrosis is extremely rare
• Not clinically significant in routine use

• Serum fluoride levels may rise slightly but are far below nephrotoxic thresholds
• Nephrotoxicity is extremely unlikely - Morgan & Mikhail 7e

• Like other halogenated agents, isoflurane can produce CO if passed through desiccated CO₂ absorbent in the anesthesia circuit (though far less than desflurane)
• Prevented by using well-hydrated, fresh CO₂ absorbent

8. SPECIAL CONSIDERATIONS

Pediatric Patients

• MAC declines from peak in infancy through childhood
• Not suitable for inhalational induction due to pungent odor
• Blood solubility is intermediate; less ideal than sevoflurane for pediatric maintenance in non-intubated patients

Elderly Patients

• MAC decreases ~6%/decade after age 40
• Reduced doses required; cardiovascular effects (hypotension) are magnified

Cardiac Patients

• Historically controversial due to coronary steal concern - now largely dismissed clinically
• Useful because cardiac output is preserved (SVR reduction mechanism)

Neuroanaesthesia

• Can be used at low concentrations (<1 MAC) with hyperventilation to maintain ICP
• The EEG burst suppression at 2 MAC is used for cerebral protection in specific scenarios

Obstetrics

• Produces uterine relaxation; use with caution for obstetric procedures to avoid excessive uterine atony and hemorrhage

9. STORAGE AND HANDLING

• Stable liquid at room temperature; does not require amber bottle or preservatives
• Non-flammable, non-explosive
• Delivered via a conventional vaporizer (unlike desflurane which requires a specialized heated, pressurized vaporizer)
• Physically stable for up to 5 years; no deterioration on sunlight exposure

KEY MNEMONICS

• Intermediate blood:gas (1.46)
• SVR reduction = mechanism of hypotension
• Output (cardiac) maintained
• Flurane - fluorinated methyl ethyl ether
• Liver: minimal toxicity (0.17% metabolism)
• Use: maintenance (not induction - too pungent)
• Renal: decreased GFR (no true nephrotoxicity)
• Alveolar concentration needed: 1.15% MAC
• Neuromuscular: relaxation + potentiates NMBDs
• EEG silence at 2 MAC (burst suppression)

• Miller's Anesthesia, 10e - Chapters 18, 72
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Chapter on Inhalational Anesthetics
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Chapter 24
• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - Chapter 18
• Lippincott Illustrated Reviews: Pharmacology - Chapter 20